1. Field of the Invention
The present invention relates to a urea-urethane grease composition, i.e. a grease composition containing a urea-urethane compound as a thickener, and particularly to such a grease composition having excellent properties and having stability with extremely little tendency to harden over a long period of time.
2. Related Art Statement
With surprising progress of heavy chemical industries in recent years, a variety of apparatuses or machines is worked under severe conditions. Under such circumstances, lubricants must have superior properties to withstand the actual conditions used. Many investigations have hitherto been made to develop a numerous variety of lubricants. However, the apparatuses or machines used in the manufacturing industries have bearings which are often exposed to relatively high temperature of higher than 150.degree. C. even under normal operating conditions. There is a case where a high loading is applied on a bearing which is operated at ultra low speed. One of the lubricants applied in-between the bearings for smooth operation of the machines is a grease composition.
Metallic soap thickeners are predominantly used as the thickeners for the greases. However, even a lithium soap grease, which is known as a universal grease, has a dropping point of about 200.degree. C. and cannot be used at a temperature range of higher than 150.degree. C.
A variety of thickeners has been proposed for use in grease compositions which can be used in a high temperature environment for a long time, the examples being various complex soaps, sodium terephthalamate, bentone and organic thickeners such as indanthrene. However, these thickeners known by the prior proposals have individual disadvantages. For example, calcium complex type compounds have a disadvantage that they tend to rapidly harden the grease. Sodium terephthalamate causes syneresis and separation of oil, or accelerates deterioration due to oxidation since it contains intramolecular metal atoms. Bentone is deterimental in lubricating property at high temperature for prolonged use, whereas indanthrene has bad hue and is expensive.
On the other hand, greases containing urea thickeners having various terminal groups include so-called diurea grease and tetraurea grease which have more advantageous merits over those used in the conventional greases described above. However, the tetraurea grease has disadvantages that the consistency thereof is decreased considerably as it is exposed to high temperature for a long time, and that it is hardened or softened, depending on the difference in shearing rate applied thereto, leading to adverse effects in practical use. Likewise, the known diurea greases containing diurea compounds having terminal groups merely composed of alkyl groups are low in dropping point to frequently separate oil at high temperature so that they cannot be used at a high temperature environment for a long time. On the other hand, the known diurea greases containing diurea compounds having terminal groups merely composed of aromatic hydrocarbon residues have a high dropping point but they are still unsatisfactory in connection with the problem of oil separation at high temperature, with a further disadvantage that the thickening capacities thereof are equivalent or even inferior to those having alkyl terminal groups. A still further problem involved in the known diurea grease containing a diurea thickener having aromatic terminal groups is that the aromatic amines used in preparation thereof have physiological toxicity to humans.
After eager investigation to overcome the problems of the aforementioned urea greases, we have found that the diurea compounds have properties well suited for use as the thickeners in greases, and that the terminal groups of the diurea compounds affect significant influence on the function of the diurea compounds. In detail, we have found a diurea compound which has superior properties when used as a thickener for a grease. The diurea compound has a cyclohexyl group or a group derived therefrom and having 6 to 12 carbon atoms or an alkyl group having 8 to 20 carbon atoms at either one of the terminal groups, the molar ratio of the cycloalkyl or derivatives thereof to the total molar equivalent of the cyclohexyl or derivatives thereof plus the alkyl group ranging from 20 to 90 mol %. The diurea grease containing the diurea compound was applied for patent and the application was published by Japanese Patent publication No. 11156/1980.
The diurea grease composition disclosed by Japanese Patent publication No. 11156/1980 has the following advantages.
(1) Change in consistency is small even after the use thereof at high temperature for a long time. PA1 (2) It has excellent mechanical stability under shearing rates varying within a wide range. PA1 (3) Separation of oil at high temperature is small. PA1 (4) It has excellent water-resistant property. PA1 (5) It exhibits powerful thickening capacity.
However, subsequent investigations on the diurea grease composition have revealed that the properties of the product fluctuate, depending on the difference in manufacturing conditions, so that the grease becomes too hard after the lapse of time when the manufacturing conditions for the product are impertinent.
Alternatively, in Japanese Laid-open Patent Publication No. 88095/1985, there is disclosed a sealing grease for a pallet sealing device of a sintering unit obtained by mixing into base oil a thickener comprised of a mixture of a diurea compound and a diurethane compound in a predetermined ratio.
As a result of our eager researches and investigations into the diurea grease disclosed in these prior-art publications, the present inventors have found that a urea-urethane grease composition obtained by mixing a diurea compound, a urea-urethane compound in which a part of the urea groups is substituted by urethane groups and a diurethane compound in a specific ratio as a thickener has highly satisfactory properties. This finding has led to fulfilment of the present invention.